This invention relates to mass spectrometers and, more particularly, to an ion beam monitor that facilitates the use of mass spectrometers.
Mass spectrometers are well known for their use in analyzing unknown samples by observing their mass spectra. To observe such mass spectra the unknown sample is first converted into an ion beam which is mass analyzed in a well-known manner. Various high energy and low energy sources are used to provide ions of the unknown sample.
In contrast to electron impact mass spectrometry (a high energy source), field desorption sources produce relatively uncomplicated mass spectra that characterize the molecular weight of various materials. The technique known as field desorption mass spectrometry has come into extensive use in the last few years, particularly for the analysis of organic compounds. Field desorption mass spectrometry utilizes stable field desorption emitters having long dendrites capable of adsorbing sufficient sample to provide useful field desorption spectra. Such field desorption emitters are described by H. D. Beckey et al., J. Physics E.; Scientific Instruments, 6, 1043 (1973).
A field desorption ion source of conventional design produces positive ions of the sample applied to the emitter. Such ions are produced when the emitter is heated in an electric field of sufficient strength, usually 10.sup.7 volts/centimeter, to remove an electron from the sample molecule, Such removal normally occurs at one of the many tips of the dendrites on the emitter. These ions are produced from the sample that is applied to the emitter when and if two conditions are simultaneously achieved. The first is that the sample remains on the emitter as the emitter is heated. Secondly, proper for ionization of the sample must exist within the temperature and electric field characteristics of the source.
In the analysis of unknown materials, neither of these conditions are known. When these uncertainties are added to the fact that the ions to be expected in the analysis are not known and the operational difficulties associated with field desorption analyses, it is imperative that the operator know when ions are being produced from the sample, irrespective of mass analysis. It would be highly desirable if one were able to first learn the field desorption characteristics of the sample and then perform the mass analysis. This would result in a great reduction of the time required.